Generally, a continuously variable transmission (CVT) is a device that continuously varies the transmission gear ratio between a maximum transmission gear ratio and a minimum transmission gear ratio according to a preset gear shaft pattern and, accordingly, allows a machine to efficiently use the engine output power, thereby realizing excellent power transmission performance and a mileage increase.
The CVTs are generally and widely used in a variety of industrial fields, such as automobiles, industrial machines, hoists, conveyors for goods transfer, winches, elevators and escalators. However, most of the conventional CVTs have a complicated construction so that they are easily disabled when being used. Further, although it is known that most conventional CVTs can steplessly or continuously vary the transmission gear ratio, the conventional CVTs practically, discontinuously varies the transmission gear ratio on an operational construction basis thereof, so that the range of varying the transmission gear ratio is limited and, accordingly, the conventional CVTs may not be efficiently adapted to a variety of uses.
In the related art, a V-belt type CVT 100 shown in FIG. 1 has been widely used as a CVT. The V-belt type CVT 100 includes a variable drive pulley 107, a variable driven pulley 114 and a V-belt 115. The variable drive pulley 107 has a drive-side fixed pulley plate 102 fixed to an outer circumferential surface of a drive shaft 101 receiving the output power of an engine, a drive-side movable pulley plate 103 movably mounted to the outer circumferential surface of the drive shaft 101 so that the movable pulley plate 103 can move in an axial direction and forms a drive-side V-groove 104 in conjunction with the drive-side fixed pulley plate 102, and a drive-side hydraulic cylinder 106 forming a hydraulic chamber 105 on a rear surface of the drive-side movable pulley plate 103. The variable driven pulley 114 has a driven-side fixed pulley plate 109, which is fixed to an outer circumferential surface of a driven shaft 108 arranged at a predetermined location spaced apart from the drive shaft 101 so that the driven shaft 108 is parallel to the drive shaft 101, a driven-side movable pulley plate 110 movably mounted to the outer circumferential surface of the driven shaft 108 so that the movable pulley plate 110 can move in an axial direction and forms a driven-side V-groove 111 in conjunction with the driven-side fixed pulley plate 109, and a driven-side hydraulic cylinder 113 forming a hydraulic chamber 112 on a rear surface of the driven-side movable pulley plate 110. The V-belt 115 is connected between the variable drive pulley 107 and the variable driven pulley 114 by wrapping both around the drive-side V-groove 104 of the drive pulley 107 and around the driven-side V-groove 111 of the driven pulley 114.
The conventional V-belt type CVT having the above-mentioned construction is configured to realize a desired transmission gear ratio by varying the radiuses of contact between the V-belt 115 and the variable drive pulley 107 and the variable driven pulley 114. Here, the variation in the radiuses of contact between the V-belt 115 and the pulleys 107 and 114 may be realized by applying a predetermined hydraulic pressure to the hydraulic chamber 105 of the drive-side hydraulic cylinder 106 through a hydraulic line formed in the drive shaft 101 so that the drive-side movable pulley plate 103 is moved in an axial direction to reduce the width of the drive-side V-groove 104 of the variable drive pulley 107 and, at the same time, by applying a predetermined hydraulic pressure to the hydraulic chamber 112 of the driven-side hydraulic cylinder 113 through a hydraulic line formed in the driven shaft 108 so that the driven-side movable pulley plate 110 is moved in an axial direction to enlarge the width of the driven-side V-groove 111 of the variable driven pulley 114. Alternatively, the variation in the radiuses of contact between the V-belt 115 and the pulleys 107 and 114 may be realized by enlarging the width of the drive-side V-groove 104 of the variable drive pulley 107 and, at the same time, by reducing the width of the driven-side V-groove 111 of the variable driven pulley 114.
However, the above-mentioned conventional V-belt type CVT is problematic in that the V-belt 115 is in frictional contact both with the variable drive pulley 107 and with the variable driven pulley 114, so that when an excessive load is imposed on the belt and pulleys, the V-belt 115 may slip over the variable drive pulley 107 and over the variable driven pulley 114 and, accordingly, the V-belt may fail to efficiently transmit the engine power and the durability of the V-belt 115 may be remarkably reduced by the slip. Further, the tension of the V-belt 115 may be easily reduced by the repeated use of the CVT, so that the V-belt may not perform power transmission as desired.